1. Field of the Invention
The present invention relates to a magnetic field detecting element, and more particularly, to the element structure of a magnetic field detecting element having a pair of free layers.
2. Description of the Related Art
A GMR (Giant Magneto-Resistance) element is known as a reproducing element (magnetic field detecting element) of a thin film magnetic head. Hitherto, CIP (Current in Plane)-GMR element, in which sense current flows in a direction that is horizontal to the film surface of the element, have been mainly used. In recent years, however, in order to realize higher recording density, elements have been developed in which sense current flows in a direction that is perpendicular to the film surface of the element. TMR elements utilizing the TMR (Tunnel Magneto-Resistance) effect and CPP (Current Perpendicular to the Plane) elements utilizing the GMR effect are known as elements of this type. In this specification, an element in which sense current flows in a direction that is perpendicular to the film surface of the element is generally referred to as a CPP-type element.
Conventionally, the CPP element includes a stack having a magnetic layer (free layer) whose magnetization direction changes in accordance with an external magnetic field, a magnetic layer (pinned layer) whose magnetization direction is fixed with respect to the external magnetic field, and a non-magnetic intermediate layer sandwiched between the pinned layer and the free layer. As a pinned layer, a so-called synthetic pinned layer, in which two magnetic layers are antiferromagnetically coupled to each other, is generally used, and an antiferromagnetic layer is provided adjacent to the synthetic pinned layer. On both sides of the stack with regard to the track width direction thereof, bias magnetic layers for applying a bias magnetic field to the free layer are provided. The free layer is magnetized into a single magnetic domain by a bias magnetic field that is emitted from the bias magnetic layers. This provides an improvement in linear change in resistance with respect to a change in an external magnetic field and an effective reduction in Barkhausen noise. The relative angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer changes in accordance with an external magnetic field, and as a result, electric resistance of sense current that flows in a direction that is perpendicular to the film surface of the stack is changed. By making use of this property, external magnetization is detected.
In recent years, higher linear recording density is desired. However, an improvement in linear recording density requires a reduction in spacing between upper and lower shield layers (a gap between shields). In recent years, novel film configurations that are completely different from conventional stacks and that have a potential to meet this requirement have been proposed. U.S. Pat. No. 7,019,371 discloses a CIP element having two free layers whose magnetization directions vary in accordance with an external magnetic field and a non-magnetic spacer layer sandwiched therebetween. U.S. Pat. No. 7,035,062 discloses a CPP-GMR element having two free layers whose magnetization directions vary in accordance with an external magnetic field and a non-magnetic spacer layer sandwiched therebetween. In these elements, the two free layers are exchange-coupled with each other due to the RKKY (Rudermann, Kittel, Kasuya, Yoshida) interaction that occurs via the non-magnetic spacer layer. A bias magnetic layer is disposed on the back side of the stack, as viewed from the air bearing surface, and applies a bias magnetic field in the direction that is perpendicular to the air bearing surface. The magnetization directions of the two free layers form a certain relative angle due to the magnetic field that is applied from the bias magnetic layer. When an external magnetic field is applied in the direction that is perpendicular to the air bearing surface from a recording medium in this state, the magnetization directions of the two free layers are changed, and as a result, the relative angle formed by the two free layers is also changed, and thereby, the electrical resistance of the sense current is changed. These properties enable detection of external magnetization. In this way, the film configuration using two free layers, which does not require a conventional synthetic pinned layer and an antiferromagnetic layer, has a simple film configuration, and has potential for reducing the shield gap.
The reason why the bias magnetic layer is provided on the back side of the stack, as viewed from the air bearing surface, in a magnetic field detecting element that uses two free layers described above is as follows: FIG. 1A is a conceptual view of the above-described magnetic field detecting element having two free layers. In FIGS. 1A to 2D which will be referred to below, only layers that are necessary for explanation are depicted. Bias magnetic layer 116 is provided on the back side of free layers 153, 155, as viewed from the air bearing surface. The arrows on free layers 153, 155 indicate the magnetization directions of the respective free layers (The broken line and the solid line indicate the magnetization direction of free layer 153 and the magnetization direction of free layer 155, respectively.). The arrow on bias magnetic layer 116 indicates the direction of a bias magnetic field. FIG. 1B is a top view of the magnetic field detecting element shown in FIG. 1A, as viewed from direction 1B-1B in FIG. 1A. Two free layers 153, 155 are exchange-coupled to each other via a non-magnetic spacer layer (not shown) sandwiched therebetween. By appropriately setting the magnitude of the bias magnetic field applied from bias magnetic layer 116, two free layers 153, 155 are magnetized such that the magnetization directions thereof form right angles while being exchange-coupled to each other. It should be noted that this magnetization state requires two free layers 153, 155 to be exchange-coupled to each other such that the magnetization directions thereof are anti-parallel with each other, i.e., exchange-coupled by a negative exchange coupling force. In the present description, “parallel” means that magnetization directions are parallel with each other and are directed in the same direction, while “anti-parallel” means that magnetization directions are parallel with each other but are directed in opposite directions.
When external magnetic field MF1 that is directed from air bearing surface ABS toward the magnetic field detecting element is applied in this state, as shown in FIG. 1C, the magnetization directions of two free layers 153, 155 are rotated toward the direction of external magnetic field MF1. As will be apparent from the figure, the magnetization directions of two free layers 153, 155 are rotated in directions that are opposite to each other so that the relative angle formed by the magnetization directions of the layers decreases as if a pair of scissors was closed. This results in a decrease in electrical resistance to the sense current. Since two free layers 153, 155 are exchange-coupled to each other, the relative angle formed by the magnetization directions of two free layers 153, 155 is a function of the exchange coupling force and the magnitude of the external magnetic field. When external magnetic field MF2 that is directed away from the magnetic field detecting element is applied in the same manner, as shown in FIG. 1D, the relative angle formed by the magnetization directions of two free layers 153, 155 increases as if a pair of scissors was opened, unlike the case of FIG. 1C. This results in an increase in electrical resistance to the sense current. In this way, electrical resistance to the sense current changes in accordance with the direction and the magnitude of the external magnetic field, and the external magnetic field can be detected.
Next, FIG. 2A is a conceptual view showing a magnetic field detecting element that includes two free layers, which are the same as the ones in FIG. 1A, and bias magnetic layers that are provided on both sides of the free layers, as viewed in the track width direction. FIG. 2B is a top view of the magnetic field detecting element shown in FIG. 2A, as viewed in direction 2B-2B in FIG. 2A. Two free layers 153, 155 are exchange-coupled to each other via a non-magnetic spacer layer (not shown) that is sandwiched therebetween. By appropriately setting the magnitude of a bias magnetic field that is applied from bias magnetic layers 216a, 216b, two free layers 153, 155 are magnetized such that the magnetization directions form substantially right angles, while being exchange-coupled to each other.
When external magnetic field MF1 that is directed from air bearing surface ABS toward the magnetic field detecting element is applied in this state, as shown in FIG. 2C, the magnetization directions of two free layers 153, 155 are rotated toward the direction of external magnetic field MF1. However, the magnetization directions of two free layers 153, 155 remain perpendicular to each other. This is because the magnetization directions of two free layers 153, 155 are rotated in the same direction in this case, and hence, no force to overcome the exchange coupling force and thereby to change the relative angle between the magnetization directions of two free layers 153, 155 is produced. For this reason, the relative angle formed by the magnetization directions of two free layers 153, 155 changes little even when external magnetic field MF1 is applied, and accordingly, a sufficient change in magnetoresistance cannot be obtained. Similarly, when external magnetic field MF2 that is directed away from the magnetic field detecting element is applied, as shown in FIG. 2D, the state in which the magnetization directions are perpendicular to each other is kept as it is. Therefore, the relative angle that is formed by the magnetization directions of two free layers 153, 155 changes little even when external magnetic field MF2 is applied, and accordingly, a sufficient change in magnetoresistance cannot be obtained.
For the reasons stated above, a magnetic field detecting element having two free layers is provided with a bias magnetic layer on the back side of the stack, as viewed from the air bearing surface. However, this arrangement is disadvantageous in that a bias magnetic field cannot be effectively applied to the free layers because this arrangement allows only one bias magnetic layer. The bias magnetic field can be effectively applied by providing a pair of bias magnetic layers on both sides of the free layers in order to concentrate a sufficient amount of magnetic field on the free layers without divergence. However, it is impossible to provide a bias magnetic layer on the side of the air bearing surface side, and no fundamental solution has been found.